CODESARRAY

Lesson 14, by Codesarray, , 4 min read

Python to-do list project for beginners: lists, dicts and JSON

Build a small Python to-do app from what you already know: tasks as a list of dictionaries, saved to a JSON file, loaded back with try and except, and printed with enumerate.

By the end of this page you will have a working Python to-do list that remembers its tasks between runs. It uses only things from the earlier lessons: lists, dictionaries, a function, a file, JSON and try/except.

The data: a list of dictionaries

Start with the data, because the program is just a way of changing it. One task is a dictionary with two keys: title, and done, which is True or False. The whole to-do list is a list of those dictionaries.

Adding a task is todos.append(...). Ticking one off means picking it by position and setting its done key: todos[0]["done"] = True. Nothing here is new. It is lists and dictionaries working together.

Save the list to a file

A list in memory disappears when the program ends, so save it. json.dumps(todos) turns the list into text, and path.write_text(...) puts that text in a file. json.loads(path.read_text()) does the reverse and gives you the list back.

JSON is a text format that Python lists, dictionaries, strings, numbers, True, False and None map onto. In the file, True is written as true and False as false.

Load it back, even on the first run

The first time the program runs, todos.json does not exist, and path.read_text() raises FileNotFoundError. Wrapping the load in a function with try and except fixes that: if the file is missing, return an empty list instead.

def load():
    try:
        return json.loads(path.read_text())
    except FileNotFoundError:
        return []

The app now starts fresh when there is no file, and remembers everything after that. Catch FileNotFoundError by name, not a bare except, so a real bug is not hidden.

The whole program

import json
from pathlib import Path
path = Path("todos.json")


def load():
    try:
        return json.loads(path.read_text())
    except FileNotFoundError:
        return []


todos = load()
todos.append({"title": "Buy milk", "done": False})
todos.append({"title": "Learn Python", "done": False})
todos[0]["done"] = True
path.write_text(json.dumps(todos))
for number, todo in enumerate(todos, start=1):
    mark = "x" if todo["done"] else " "
    print(f"{number}. [{mark}] {todo['title']}")

Lines 1 and 2 import json and Path. Line 3 points at todos.json. Lines 6 to 8 define load, which tries to read and parse the file, and lines 9 and 10 catch FileNotFoundError and return an empty list. Line 13 calls load. Lines 14 and 15 append two tasks, and line 16 marks the first one done. Line 17 saves everything to the file. Line 18 loops over the tasks with enumerate, starting the count at one. Line 19 picks an x or a space, and line 20 prints each task.

Running it prints:

1. [x] Buy milk
2. [ ] Learn Python

The file on disk holds the same list, ready for the next run.

What you learned

  • A task is a dictionary, and a to-do list is a list of those dictionaries.
  • json.dumps and write_text save a list, and read_text and json.loads bring it back.
  • try and except FileNotFoundError let a program start when its file does not exist yet.
  • enumerate(todos, start=1) gives each item a number for printing.

Common questions

**Why json.dumps and write_text instead of json.dump?** Both work. dumps returns a string, so it pairs with Path.write_text and keeps the program short. json.dump writes straight to an open file.

**Why does the file show true instead of True?** JSON has its own spelling. Python's True, False and None are written true, false and null, and json.loads converts them back.

**What happens if todos.json is empty?** json.loads("") raises json.JSONDecodeError, which is a different error from FileNotFoundError, so this load would not catch it. Delete the file or write [] into it.

Why do the tasks double every time I run it? Each run loads the saved list and then appends the two tasks again. A real app would read each task from the user, for example with input(), instead of writing them into the program.

Where do I go from here? Add a delete command, a due date key on each task, or a function for each action. Those are the same lists, dictionaries and functions, used a little more.

That finishes the Python beginner track. Next in the series: the Intermediate track begins with Big O in Python, and how to measure what your code costs.

This article goes with the video Build a to-do app. Watch it on YouTube.